In recent years, morbidity of colon cancer is increasing with the westernization of diets in Japan. It is said that high-fat diets and low-fiber diets change the intestinal microflora and increase production of carcinogens, and further, reduction of feces amount prolongs intestinal feces retention time and hence prolongs contact time between carcinogens and the intestinal canal, resulting in increase of risks of oncogenesis. Bacteria in the intestine extremely closely relate to health and diseases of hosts and considered to associate with colon cancer through diet components and body components.
Streptococcus (St..) bovis is known as a pathogenic bacterium of infectious endocarditis, and high concurrent incidence of infectious endocarditis due to this bacterium and colon cancer attracts attention in Europe and United States (Honberg P. Z. et al., Lancet, i: 163–164, 1987). Meanwhile, Osawa et al. isolated a bacterium having a tannase activity, which hydrolyzes an ester bond in tannic acid to release gallic acid, from feces of koala eating eucalyptus containing tannin at a high concentration, and identified this bacterium as St. bovis biotype I (Osawa R. et al., Appl. Environ. Microbiol., 56: 829–831, 1990). This bacterium has the tannase activity for degrading tannin to release gallic acid, and contains decarboxylase that decarboxylates gallic acid into pyrogallol, and it is proposed that it should be newly designated as St. gallolyticus. Osawa inferred a possibility that this St. gallolyticus was a bacterium identical to St. bovis isolated from a colon cancer patient concurrently having infectious endocarditis (Osawa R., RIKEN Symposium Abstracts, “Classification and Ecology of Lactic Acid Bacteria”, pp. 36–45, 1996).
In general, a substance referred to as tannin belongs to polyphenols, natural substances constituting an important portion of phenolic compounds, and has toxic and growth-inhibitory actions on microorganisms as well as an astringent taste. While various classifications of tannin have been proposed, tannin is largely classified into hydrolysable tannin and condensed tannin for convenience. The former is structurally pyrogallol tannin, and it is also referred to as pathological tannin since it is contained in nutgall or gall nut in a large amount. On the other hand, the latter is catechol tannin, and it is also referred to as physiological tannin since it is a normal component of plants. The hydrolysable tannin has a chemical structure in which phenolic acid (gallic acid, ellagic acid etc.) binds to a saccharide as a core through an ester bond. It is known that this hydrolysable tannin is hydrolyzed by tannase, which is a tannin-degrading enzyme mainly produced by fungi such as Aspergillus and Candida living in soil. However, production of tannase by bacteria living in the intestinal canal of animals had not been reported before the publication of Osawa.
Since then, it has been reported that Lonepinella koalarum was isolated from feces of koala as a tannase-positive bacterium, in addition to St. gallolyticus, and also isolated mainly from herbivorous animals (Osawa R., Syst. Appl. Microbiol., 15: 144–147, 1992). Recently, it has been further reported that Lactobacillus (L.) plantarum was isolated from human feces. L. plantarum is a lactobacillus mainly isolated from silos, and infectious endocarditis due to L. plantarum has also been reported (Oakey H. J. et al., J. Appl. Bacteriol., 78: 142–148, 1995).
The onset mechanism of colon cancer has been studied by many researchers, and existence of active oxygen can be mentioned as a part of the mechanism (Babbs C. F. et al., Free Rad. Biol & Med., 8: 191–200, 1990). Active oxygen exhibits a sterilizing action against microorganisms entering into a living body to protect the body from infection. On the other hand, active oxygen has a risk of increasing adverse reaction products that can damage body functions by radical chain reactions and thus worsening various pathological conditions. It is considered that balance of production and elimination of active oxygen is lost in, in particular, aging and life habit diseases such as arteriosclerosis and cancer, leading to worsening of conditions over a long time.
Many chemical substances have been discovered as substances producing active oxygen, and gallic acid and pyrogallol are among them (Khan N. S., Mutagenesis, 13: 271–274, 1998).